JP2695891B2 - Comparator circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a comparator circuit using a field-effect transistor (hereinafter, simply referred to as an FET or a transistor), and particularly to an N-channel FET or a P-channel FET. The present invention relates to an improved gain of a comparator circuit using only one type.

[Prior Art] FIG. 2 shows a basic configuration example of a conventional comparator circuit.

The transistor Q ₂₁ and Q ₂₃ are formed in the same size for the same characteristics. Further, in order to form a differential amplifier in the transistors Q ₂₁ and Q _23, these sources are connected to the drain of the transistor Q ₂₅ of the constant-current source connected in common. Source and gate of the transistor Q ₂₅ is connected is short-circuited to the power supply voltage V _SS.

The drains of the transistors Q ₂₁ and Q ₂₃ are commonly connected to the power supply voltage V _DD via active load transistors Q ₂₂ and Q ₂₄ , respectively.

The signal voltage V ₂ in the gate of the transistor Q _21, a reference voltage V ₂ ref is input to the gate of the transistor Q _23,
It has become as from the drain of the transistor Q ₂₁ taking out the output V ₂ out.

Therefore, when the signal voltage V ₂ in is made larger than the reference voltage V ₂ ref, the transistor Q ₂₁ works in a direction in which more current flows than the transistor Q _23, and the current flowing in the active load transistor Q ₂₂ becomes smaller. The output voltage V ₂ out decreases.

On the other hand, when the signal voltage V ₂ in is made smaller than the reference voltage V ₂ ref, the transistor Q ₂₃ works to flow more current than the transistor Q ₂₁ and flows to the active load transistor Q ₂₄ . The current increases and the output voltage V ₂ out increases.

Therefore, the present circuit performs a comparator operation by increasing the gain of the differential amplifier.

[Problems to be Solved] However, in the conventional comparator circuit of the above-described configuration, when increasing the signal voltage V ₂ in comparison with the reference voltage V ₂ ref, by the increase in current of the transistor Q ₂₁ of Q ₂₃ It operates to reduce the current. That decrease in the current of the transistor Q _23, since the transistor Q ₂₃ is operating in a non-saturation region, the gate of the transistor Q ₂₃ - source voltage Vgs
So as to reduce the, increase the source potential V ₂ s of the transistor Q ₂₃ and Q _21.

That is, when the signal voltage V ₂ in is higher than the reference voltage V ₂ ref, the amount of increase in the gate-source voltage Vgs of the transistor Q ₂₁ is larger than the amount of increase in the signal voltage V ₂ in by the transistor Q 2.
It will be reduced by the decrease of ₂₃ Vgs.

This is because the transistor Q ₂₁ and Q ₂₃ characteristics are the same, the amount of change in V _gs of the transistor Q ₂₁ means that decreases the just half of the signal voltage V _2in.

Here, the principle will be described by a first-order approximation of the differential amplifier. It is assumed that the signal voltage is changed by ΔV _2in . The amounts of change in the gate-source voltages of the transistors Q ₂₁ and Q ₂₃ are ΔV _gs21 and ΔV _gs23, and the amounts of change in the drain-source current are ΔI _ds21 and ΔI _ds23 . Since the transistor Q ₂₁ and Q ₂₃ characteristics is the same, the transconductance g is the same.
Then _ΔI ds21 is composed of the product of the _ΔV gs21 and the mutual conductance _g, ΔI ds23 is the product of the _ΔV gs23 and the mutual conductance g.

ΔI _ds21 = ΔV _gs21 × g (1) ΔI _ds23 = ΔV _gs23 × g (2) The sum of the currents flowing through the left and right transistors of this differential pair is determined by the transistor Q which is a constant current source. Since this is equal to ₂₅ drain currents, the sum of the changes in the current flowing through the left and right transistors is zero. That is, the sum of ΔI _ds21 and ΔI _ds23 becomes zero.

ΔI _ds21 + I _ds23 = 0 (3) Further, the amount of change in the source potential of the transistors Q ₂₁ and Q ₂₃ is ΔV
_{If 2s} , ΔV _gs21 is the sum of ΔV _2in and ΔV _2s , and ΔV
_gs23 becomes ΔV _2s .

ΔV _gs21 = ΔV _2in + ΔV _2s (4) ΔV _gs23 = ΔV _2s (5) When these relationships are arranged into a relationship between ΔV _gs21 and ΔV _2in , ΔV _gs21 is equal to ΔV _2in / 2. Recognize.

That is, when the expressions (1) and (2) are substituted into the expression (3), ΔV _gs21 × g + ΔV _gs23 × g = 0 (6) Therefore, ΔV _gs23 = −ΔV _gs21 (7) ( From equations (5) and (7), ΔV _2s = ΔV _gs23 = −ΔV _gs21 ... (8) By substituting equation (8) into equation (4), ΔV _gs21 = ΔV _2in = −ΔV _gs21. 9) Therefore, ΔV _gs21 = ΔV _2in / 2.

Therefore, in this circuit can not sufficiently exhibit the ability to switch the transistor Q _21, the gain of the comparator circuit has a drawback that decreases.

In order to improve the gain, there are methods of increasing the load resistance and increasing the transconductance of the transistor.

For example, to increase the load resistance, the pinch-off voltage Vp =
Some use −2V transistors (D. Meignant
et al .; “A High Performance 1.8GHz Strobed Compar
ator For A / D Converter ", IEEE GaAs IC Sympo.Tech.Di
gest. pp. 66-69, Oct. 1983).

However, as long as a differential amplifier is used, the gate-source voltage Vgs on the input side is unavoidably fluctuated by the input, so that the above-described disadvantage cannot be essentially improved.

An object of the present invention is to solve the above-described drawbacks of the prior art by making the source voltage of the input-side transistor constant regardless of the input, so that even when the signal voltage becomes larger than the reference voltage, It is an object of the present invention to provide a comparator circuit which can sufficiently exhibit the switching characteristics of the transistor without decreasing the gate-source voltage of the transistor and obtain a large gain.

[Means for Solving the Problems] A comparator circuit according to the present invention includes a reference transistor having a gate to which a reference voltage is applied, and a drain of the transistor is directly or through a low resistance load to a voltage source.
Sources are connected to current sources, respectively, to form a source follower circuit having the source as an output.

On the other hand, an input-side transistor having a gate to which a signal voltage is applied has a drain connected to the voltage source via an active load, a source connected to the output of the source follower circuit, and a drain connected to the output. A single-ended circuit.

Further, the transconductance of the reference-side transistor is made larger than that of the input-side transistor.

[Operation] Even if the signal voltage applied to the transistor on the input side of the single-ended circuit fluctuates and the current flowing in the reference-side transistor of the source follower circuit changes, the output impedance is small, so that the source follower circuit is connected to the source follower circuit. The source of the input-side transistor is not accompanied by voltage fluctuation, and a voltage equal to the reference voltage always appears at the source potential.

Therefore, since the in-phase change of the source potential of the input transistor becomes small, even if the signal voltage becomes higher than the reference voltage, the gate-source voltage of the input transistor does not decrease, and The switch characteristics are fully exhibited.

Embodiment An embodiment of the present invention will be described below with reference to FIGS. 1 and 3.

FIG. 1 shows an example of the comparator circuit of the present invention. Here, the transistor constituting the comparator circuit is N
Any of a channel type and a P-channel type may be used.

Input-side transistor Q ₁₁ to gate the signal voltage V ₁ in is input, its drain is connected to the power supply voltage V _DD via the active load transistor Q ₁₂ which is short-circuited between the gate and the source, the drain of the transistor Q ₁₁ Is used as an output terminal. The source of the transistor Q ₁₁ is connected to the source follower circuit B will be described later.

Referring side transistor Q ₁₃ to reference voltage V ₁ ref is input to the gate is configured to connect the drain directly to the power V _DD, by connecting the constant current source transistor Q ₁₄ to the source, the source of the transistor Q ₁₃ Are configured as a source follower circuit B which outputs the following. Constant current source transistor Q ₁₄
It is short-circuited between the gate and the source, shorted gate, and a source connected to a power supply voltage V _SS.

To configure the source follower circuit B, the reference voltage V
The value of the drain current of the transistor Q ₁₃ by gate application of _1ref, set larger than the value of the constant current source transistor Q _14, the source-drain voltage across transistor Q ₁₃ is set high, the transistor Q ₁₃ I- From the viewpoint of the V characteristic, the operation (that is, overdrive) in the saturation region is performed.

Thus, the transistor Q ₁₃ is not a non-saturation region, it will operate in the saturation region. The transistor Q ₁₃
If but to operate in the saturation region, and the transistor Q ₁₃ and the drain and the source voltage V _DD, rather than the direct connection as in the illustrated example, it may be interposed a low resistance between them.

The transistor Q constituting the single-ended circuit A
₁₁ source is connected to a connection point between the constant current source drain of the transistor Q ₁₄ and the source of the transistor Q ₁₃ of the source follower circuit B.

The above-mentioned transistors Q ₁₁ , Q ₁₂ , Q ₁₃ , Q ₁₄ are all made to have the same threshold voltage Vt.

Also, a transistor constituting the single-ended circuit A
Q ₁₁ and Q ₁₂ each other in the same gate width W _1, the source follower circuit B
In the transistor Q ₁₃ and Q ₁₄ to each other also the same gate width W ₂
To are designed respectively, W ₂ as compared with W ₁ is are the sufficiently large value.

Thus, the transistor Q ₁₃ of the source follower circuit B,
To the gate width W ₂ of Q ₁₄ to a value greater than the single-ended circuit B is to reduce the output impedance of the source follower circuit B by increasing the transconductance.

 Now, the operation of the above configuration will be described.

When a signal voltage V ₁ in larger than the source potential V ₁ s of the transistor Q ₁₁ is applied to the gate, the active load transistor Q
Transistor Q ₁₁ compared to ₁₂ is turned on, decreases to up to power supply voltage V _DD output voltage V ₁ out was near the value V ₁ s it.

Conversely, the source potential V ₁ small signal voltage V ₁ in than s of the transistor Q ₁₁ is applied to the gate, the transistor Q ₁₁ compared to active load transistor Q ₁₂ is turned off, the output voltage V ₁ out almost Power supply voltage rises to V _DD .

That is, the single-ended circuit A including the transistors Q ₁₁ and Q ₁₂ Metropolitan performs comparator operates as a reference voltage V ₁ s.

On the other hand, the source follower circuit B composed of the transistors Q ₁₃ and Q _14, by the gate width W ₂ of the transistor of the source follower circuit B are to a larger value than the single-ended circuit A, the output impedance transistor Q ₁₁ since sufficiently small as compared with, also the input-side transistor Q ₁₁ is carried out on / off operation, the source potential V ₁ s hardly changes, retains substantially the same potential as the reference voltage V ₁ ref .

Therefore, when increasing the signal voltage V ₁ in comparison with the reference voltage V ₁ ref, the gate of the transistor Q ₁₁ - increase in the source voltage Vgs without decreases with increase of the signal voltage V ₁ in, Increase as it is.

This switching capability of the transistor Q ₁₁ is exhibited regrettable without means to significantly increase the gain of the comparator circuit.

FIG. 3 shows a simulation example of the input / output characteristics of the present embodiment by a circuit simulator. Power supply voltage V _DD = 0V, V
_{Apply SS} = 5V, reference voltage V ₁ ref = −2V, and gate width ratio W ₂ / W
₁ = 5 is set.

It can be seen that the circuit of the present embodiment has a higher gain than the conventional circuit shown in FIG. According to the figure, the maximum gain is about 1.9 times larger than that of the conventional circuit, and an essential improvement as theoretically observed.

According to the present embodiment as described above, rather than the double-ended circuit of the differential comparator circuit, single ended circuit constituted by A and source follower circuit B, the input side of the transistor Q ₁₁ of the single-ended circuit A Is connected to the source follower circuit B, so that the source potential of the input-side transistor becomes almost constant even if the signal voltage fluctuates. Therefore, even when the signal voltage V ₁ in is higher than the reference voltage V ₁ ref, the gate of the transistor Q ₁₁ - source voltage Vgs has never decreases with increase of the signal voltage V ₁ in. Therefore, a large gain can be obtained by sufficiently exhibiting the switching characteristics of the transistor.

In the above-described embodiment, the case where the gate width of the transistor constituting the source follower circuit is increased is described. However, the present invention is not limited to this, and it is simply a means for increasing the mutual conductance of the source follower circuit. Any of them can be used.

According to the present invention, a circuit for comparing a signal voltage with a reference voltage is constituted by a single-ended circuit and a source follower circuit, and the source of the input-side transistor of the single-ended circuit is connected to the source follower circuit. In addition, when the signal voltage fluctuates, the change in the in-phase of the source potential becomes small, the switching characteristics of the transistor can be sufficiently exhibited, and a comparator circuit having a high gain can be obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a comparator circuit according to the present invention, FIG. 2 is a circuit diagram of a conventional example, and FIG. 3 is an explanatory diagram showing a simulation example of input / output characteristics of the present embodiment together with the conventional example. It is. Q ₁₁ is the input-side transistor, Q ₁₂ is the active load transistors, Q ₁₃ is the reference side transistor, Q ₁₄ is a constant current source transistor, A is single-ended circuit, B is the source follower circuit, V _DD is a power supply, V ₁ out output Voltage.

Claims (1)

(57) [Claims] 1. A reference transistor having a gate to which a reference voltage is applied, the drain of which is connected to a voltage source via a direct or low-resistance load, and the source is connected to a current source. A source-follower circuit as an output is formed, and the transistor has an input-side transistor to which a signal voltage is applied to a gate. A single-ended circuit having the drain as an output, wherein the transconductance of the reference transistor is larger than that of the input transistor.